Styrene-based thermoplastic elastomers have rubber elasticity at room temperature, exhibit excellent physical properties such as flexibility and moldability, have low specific gravity, and also exhibit excellent recyclability. Therefore, in relation to problems such as environmental pollution, such elastomers are recently being used as a substitute for vulcanized rubber and polyvinyl chloride in various areas such as vehicle parts, industrial parts, sundry goods, and sport applications.
Among the styrene-based thermoplastic elastomers, styrene-butadiene-styrene (SBS) block copolymers, styrene-isoprene-styrene (SIS) block copolymers, and hydrogenated products thereof are widely used because of their low cost, excellent flexibility, rubber elasticity, recyclability, and other properties.
Studies have been made to improve various properties of styrene-based thermoplastic elastomer compositions, and the following compositions (1) to (8), for example, have been proposed:
(1) a composition containing a polyolefin-based resin and a hydrogenated product of a block copolymer including a polymer block mainly composed of α-methylstyrene and a polymer block composed of isoprene and/or butadiene, the composition having a good balance of heat resistance and flowability (see Patent Document 1);(2) a composition containing a softener, an acrylic polymer, and a hydrogenated product of a block copolymer including a polymer block mainly composed of α-methylstyrene and a polymer block composed of isoprene and/or butadiene, the composition being excellent in scratch resistance and wear resistance (see Patent Document 2);(3) a thermoplastic resin composition containing a polyolefin-based resin, an acrylic resin, a hydrocarbon-based softener, a hydrogenated product of a thermoplastic block copolymer that includes a block composed of aromatic vinyl monomers and a block composed of isoprene and/or butadiene, and a hydrogenated product of a thermoplastic block copolymer that has a polymer of acrylic monomers in its side chain and includes a block composed of aromatic vinyl monomers and a block composed of isoprene and/or butadiene, the composition being excellent in flexibility and weather resistance and being capable of providing good appearance (see Patent Document 3);(4) a thermoplastic elastomer resin composition for powder molding, containing: a styrene-based thermoplastic elastomer, a softener, a peroxide-decomposable olefin-based resin, an unsaturated glycidyl compound, an unsaturated carboxylic acid, a composition obtained by dynamic crosslinking of liquid polybutadiene, and at least one material selected from the group consisting of polyester polymers, polyurethane polymers, and polyamide polymers; wherein the thermoplastic elastomer resin composition is obtained by kneading the composition obtained by the dynamic crosslinking with a compound having a specific structure or by adding the compound having the specific structure before the dynamic crosslinking; the elastomer resin composition being used to obtain a molded product having a soft touch and good scratch resistance (see Patent Document 4);(5) a thermoplastic resin composition for a golf ball, containing a styrene-based thermoplastic elastomer, a softener, a peroxide-crosslinkable olefin-based resin, a peroxide-decomposable olefin-based resin, and an organic peroxide, the composition having excellent moldability and wear resistance (see Patent Document 5);(6) a thermoplastic elastomer composition containing an ethylene-based copolymer, a styrene-based thermoplastic elastomer, and other components, the composition being excellent in flexibility, wear resistance, lightweight properties, and mechanical strength properties (see Patent Document 6);(7) a resin composition for powder molding, including a polypropylene-based resin, an olefin-based thermoplastic elastomer, a styrene-based thermoplastic elastomer, and a silicone oil-containing olefin-based resin, the composition being excellent in wear resistance and releasability after powder molding (see Patent Document 7); and(8) a composition including an ethylene-α-olefin-based copolymer rubber, a propylene-based resin, an aromatic vinyl compound-conjugated diene compound copolymer rubber having a specific structure, and a silicone oil or silicone rubber, the composition and a molded product thereof being excellent in wear resistance and scratch resistance (see Patent Document 8).
The above composition of Patent Document 1 has excellent heat resistance and flowability, but its wear resistance is not fully satisfactory. The compositions of Patent Documents 2 and 3 are flexible and have excellent moldability and transparency while the surface characteristics such as surface hardness, weather resistance, transparency, and other properties of the acrylic-based resins are maintained. However, since the acrylic-based resins have hygroscopicity, a drying step or a similar step is required before molding, and therefore the molding process is complicated. Patent Document 3 discloses the evaluation results of pencil scratch test performed according to JIS K5400 on the proposed compositions. However, the evaluation results for scratch resistance and wear resistance are not fully satisfactory.
Patent Document 4 discloses the results of hardness evaluation performed according to JIS K6253 on the proposed compositions. However, the evaluation results for scratch resistance and wear resistance are not fully satisfactory, and the hydrolysis resistance and weather resistance are not sufficient. Therefore, the molded products made of these compositions have problems such as a reduction in performance and yellowing. Patent Document 5 discloses the Taber abrasion values of the proposed compositions that relate to wear resistance, but the values thereof are not fully satisfactory. In addition to this, these compositions require dynamic crosslinking and therefore are inferior in moldability.
Patent Document 6 discloses the DIN abrasion values of the proposed compositions that are used for wear resistance evaluation, but the values thereof are not fully satisfactory. Patent Documents 7 and 8 disclose the evaluation results of the properties of the molded surfaces of the proposed compositions before and after abrasion test using an unbleached muslin No. 3 cloth, but the results of wear resistance evaluation are not fully satisfactory. Moreover, the silicone oil and silicone rubber cause a bleeding problem on the molded surface.    Patent Document 1: Japanese Patent Application Laid-Open No. 2004-91531    Patent Document 2: Japanese Patent Application Laid-Open No. 2004-2657    Patent Document 3: Japanese Patent Application Laid-Open No. Hei 5-345841    Patent Document 4: Japanese Patent Application Laid-Open No. 2001-158812    Patent Document 5: Japanese Patent Application Laid-Open No. 2002-119614    Patent Document 6: Japanese Patent Application Laid-Open No. Hei 11-335498    Patent Document 7: Japanese Patent Application Laid-Open No. 2004-231820    Patent Document 8: Japanese Patent Application Laid-Open No. 2002-173562